Differential pressure HIP forging in a controlled gaseous environment

ABSTRACT

Apparatus and procedures are presented for forging, or hot working bulk ceramics, including high temperature superconductors and other sensitive materials, under precisely controlled conditions of pressure, temperature, atmospheric composition, and strain rate. A capsule with massive end plates and an independent gas supply is located in a modified hot isostatic press (HIP). Essentially uniaxial deformation of a pre-compacted disc with forces of up to 500,000 Newtons (50 tons) and at temperatures of up to 1000 C. can be achieved. The separate gas supply to the capsule can maintain a specified gaseous atmosphere around the disc, up to the operating pressure of the HIP. The apparatus is designed to tolerate partial oxygen pressures of up to 20%.

GOVERNMENT RIGHTS

The invention described herein arose in the course of Phase II SmallBusiness Innovative Research (SBIR) Contract F49620-91-C-0065 betweenthe Air Force Office of Scientific Research and CryoPower Associates, asole proprietorship owned by the inventor. The work was sponsored by theStrategic Defense Initiative Organization (SDIO).

BACKGROUND--FIELD OF INVENTION

This invention relates to the uniaxial deformation or forging ofthermodynamically unstable, sensitive, or reactive solids, such as oxideceramics and high-temperature superconductors, under fully controlledconditions of the gaseous chemical environment, pressure, temperature,and strain rate.

BACKGROUND--DESCRIPTION OF PRIOR ART

The present invention draws on a number of overlapping arts inchemistry, metallurgy, and physics. In particular, I apply methods andconcepts from the fields of materials deformation and forging, from HotIsostatic Pressing (HIP) technology, from the thermodynamics of phasetransformations and of phase stability, and from the emerging technologyof high temperature superconducting ceramic oxides. My objective is todeform solid materials at elevated temperatures and thereby createcrystallographic texture in the material, while at the same timemaintaining the material in thermodynamic equilibrium with itsenvironment.

The problem of maintaining equilibrium conditions is particularly acutewith high temperature superconducting oxides, since most exhibitmeasurable oxygen vapor pressures at elevated temperatures. Therefore,their equilibrium chemical composition is a function of their gaseousenvironment, and the surrounding oxygen pressure determines whether agiven composition is in a thermodynamically stable state. To subject anyof these materials to deformation, or even simple compaction, withoutcontrolling their oxygen environment, will put them in athermodynamically unstable state and they will begin to decompose. Thekinetics of such decomposition depends on many factors, but the safeapproach is to not allow these materials to be in an environment wherethey can be unstable.

My invention differs from prior art in that I either use apparatus withsimilar features to attain totally different objectives, or usedifferent methods and apparatus to achieve comparable objectives.

Hot Pressing

Uniaxial hot pressing, hot deformation, or forging of a solid betweenanvils or between punches in a die will cause most materials to flow anddevelop crystallographic textures. Textured materials often exhibitimproved mechanical, electrical, or magnetic properties. Uniaxial hotpresses can be used to apply a force to a workpiece located inside afurnace that uses either a vacuum or an inert atmosphere. Graphite orceramic rods transmit the force from a hydraulic press at ambienttemperature to the workpiece. Commercially available equipment is alwayslimited in the force that can be applied by the press, typically of theorder of 20 tons. The rate or speed with which the force can be appliedis limited by the size and cost of the hydraulic compressors and theamount of travel of the punch or anvil.

Much greater capacities and speeds are available with mechanicallyoperated forges, acting on a pre-heated workpiece and deforming it in afraction of a second. However, forges of this kind are generally unableto provide a controlled or inert gaseous atmosphere around theworkpiece. This means that the workpiece must be sealed in a can.Another limitation of such machinery is that it is not possible orpractical to slow down the rate of deformation. Even greater forces andspeeds are available with explosive compacting and forging, but the samelimitations apply to that technology.

By contrast, a conventional hot isostatic press (HIP) is typically usedto consolidate or compact powdered or porous solids in a uniform orisostatic manner. The workpiece is often encapsulated or canned, so thata surrounding fluid can be used to apply uniform (isostatic) pressure toall surfaces of the workpiece without penetrating into the pores. Thecanned workpiece is located inside a furnace, which in turn issurrounded by a thermal shield, and the entire assembly is containedwithin an externally cooled pressure vessel. Pressures up to 300 MPa(3000 bar or 45,000 psi) and temperatures up to 1500 C., or higher, arereadily accessible and easily controllable. However, the size and costof the available gas compressor usually limits the rate ofpressurization to tens of minutes. Whereas uniaxial, non-isostaticcompaction in a heated die, i.e. hot pressing, is tonnage- orforce-limited, HIP procedures are pressure-limited.

A die insert inside a HIP can be used to convert isostatic into uniaxialforces. U.S. Pat. No. 5,063,022 to Zick (Dec. 5, 1991) and itscontinuation U.S. Pat. No. 5,154,882, Zick (Oct. 13, 1992) discuss a`hermetically sealed` die and punch arrangement. These patents areactually a variation of U.S. Pat. No. 5,057,273, Hanson (Oct. 15, 1991)on Uniaxial Compaction in a Cold Isostatic Process and were all assignedto the same company (Industrial Materials Technology, Inc.)

All three of these patents require close mechanical tolerances andcareful attention to and reduction of the frictional drag between thedie walls and the punch ends. They also demand great care in avoidingthe acknowledged possibility of tearing the container that encapsulatesthe punch-and-die assembly at the point where it is stretched across thedie corners. All of them fail to teach how to install afriction-reducing graphite foil at the slip plane without crinkling andthus jamming the die.

Hot Isostatic Press with Multiple Regions

The use of hermetically sealed and welded capsules is common practice inHIP technology and goes back to its very beginnings. A recent U.S. Pat.No. 5,147,086, issued to Fujikawa et al. (Sep. 15, 1992) notes earliercapsule patents, and is herewith attached by reference.

My invention makes use of an independently pressurized capsule within alarger surrounding HIP vessel. There are several patents in the HIP art,that either use such an arrangement, or discuss the problems ofindependently manipulating the fluids in an enclosure within a HIPvessel.

Gripshover, Boyer, and Harth in U.S. Pat. No. 3,633,264 (Jan. 11, 1972),entitled "Isostatic Forging," plastically deform a balloon-shapedspherical cavity inside a HIP vessel. The cavity has an independent gassupply and it is deformed by increasing the pressure on one of itssurfaces (say, the interior one) relative to the opposing (exterior)surface. The object of these inventors is in isostatically deforming thecavity or capsule itself, rather than in uniaxially deforming itscontents. Gripshover et al. do not teach how to separately manipulatethe gas pressures on the opposing surfaces.

Controlled Atmospheres

Hot pressing or HiPing in an environment of controlled gaseouscomposition or chemical activity is discussed in a number of patents:

Conaway in U.S. Pat. No. 5,137,663 (Aug. 11, 1992) claims a deformableand reusable can mounted on a plug within a HIP vessel and with anindependent gas or vacuum connection to the outside of the vessel. Hisprimary goals are speed and reusability of the can. He mentionsevacuation and the possible introduction of special processing gases, asmay be required for various workpieces. He also recognizes and discussesthe problem of keeping the elastomeric seal between the can and the plugcold. Conaway has no interest in anything other than isostaticcompaction, nor would his capsule design readily adapt to uniaxialdeformation of a workpiece. He does not discuss the issues of separatelymanipulating any high pressure gas inside the can.

A very different purpose is served by Larker's invention, U.S. Pat. No.4,152,111 (May 01, 1979). He discloses a HIP furnace with a separateinternal chamber to encapsulate radioactive materials in ceramics, whilelimiting the possible extent of radioactive contamination. He alsodescribes a dual gas and pressure supply and procedures to maintain thepressures in the two spaces at substantially the same levels. Hisprocedures are, indeed, designed to never permit significant pressuredifferences between the two spaces. The expense and complexity ofoperating separate compressors is justified by the contaminationproblems inherent in his application.

Oxygenation Atmospheres

Many oxide ceramics, including high temperature superconductingcompositions, require specific oxygen pressures to be thermodynamicallystable. These pressures range from sub-atmospheric to tens of MPa(hundreds of bars). A number of patents disclose methods and apparatusto provide oxygen atmospheres for synthesis and simple compaction. (Oursearch has excluded the many patents that use a HIP simply forsynthesizing materials without addressing the issue of oxygen control;nor have we included work on controlling the oxygen partial pressurewhen the total pressure is near, or below, atmospheric).

Morris in U.S. Pat. No. 5,130,104 (Jul. 14, 1992) and its continuationU.S. Pat. No. 5,244,638 (Sep. 14, 1993) discloses the design of acommercially available high pressure oxygen furnace, capable ofoperating at 40 MPa (400 bar) and 1000 C. He discusses the advantagesand disadvantages of externally heated furnaces vs. internally heatedHIPs. He also addresses the obvious safety hazards associated withreactive high pressure oxygen gas and with high temperature creep ofmost metals by careful selection of alloys and by numerous other designfeatures. However, his apparatus is primarily designed for laboratoryscale synthesis of new compositions and for the measurement of somephysical properties, without any provision for compacting the samples.Nor can his equipment be scaled safely to working volumes larger thanthe 10 mm diameter that is provided.

Similarly the Oxygen HIPing method disclosed by Sakurai et al. in U.S.Pat. No. 5,318,745 (Jun. 07, 1994) is essentially a process forsynthesizing a number of superconducting compositions. The cryogenicoxygen HIP proposed by Conaway in U.S. Pat. No. 4,942,750 (Jul. 24,1990) as a rapid production scheme, would be similarly limited.Unfortunately, the inventor does not address or provide any solutionsfor the numerous cryogenic engineering, materials, and safety problemsthat his design would encounter. This includes the issues of dimensionalchanges, of embrittlement, and of the violent reactivity of most hotmetals with oxygen.

The idea of placing an oxygen donor inside a HIP capsule could be asimple way to provide oxygen at the proper instant or temperature duringa fabrication process. Two very similar patents Heide et al. in U.S.Pat. No. 5,045,525 (Sep. 03, 1991) and Benfer and Richards in U.S. Pat.No. 5,059,584 (Oct. 22, 1991) specify barium peroxide as the preferreddonor. Different methods are used to physically isolate the workpiece.Heide et al. use an elaborate scheme of ceramic (boron nitride) coating,together with buffer regions of intermediate compositions as dictated bydetails of the phase diagrams, whereas Benfer simply uses a barrier ofhigh purity silica cloth. Takeshita et al. in U.S. Pat. No. 5,145,835(Sep. 09, 1992) take a different course by admixing various oxides withthe superconducting precursor materials in their HIP fabricationprocess.

In outlining how to approach the oxygenation-deoxygenation problemseveral years ago, I also reviewed a number of possible oxygen donors,including oxides, peroxides and permanganates. I reached the conclusionthat most materials should be kept from mixing with the superconductor,since this would most likely be deleterious to its superconductingproperties. I furthermore concluded that one should expect an entirelydifferent performance from materials that lose oxygen reversibly uponheating and then take it up again upon cooling, as compared to materialsthat simply decompose irreversibly. It also became evident that eachoxygen generating compound would perform differently and that a capsulewith an externally controllable oxygen supply would provide a muchgreater latitude in varying the operating conditions.

A somewhat related patent disclosure, Primdahl Iversen and Henriksen,U.S. Pat. No. 5,319,843 (Jan. 14, 1994) possibly attains pressures andtemperatures equivalent to HIP conditions, but no specific values arereported. These inventors propose to manufacture a superconducting wireby swaging a composite tube. The tube is connected to an oxygen gassupply and has porous powder-filled channels to assure that the gas canreach a central superconducting core, during both manufacture andsubsequent service.

To round out the oxygenation patents, we note a unique disclosure byEddy et al., U.S. Pat. No. 3,732,056 (May 08, 1973). These inventors usean electrolytic technique to apply partial oxygen pressures of up to 40MPa (400 bar) at 1000 to 1200 C. to a workpiece inside a uniaxial hotpress. At the temperatures of interest and with the properly appliedvoltages, a semiconducting zirconia die can be made to pump oxygen ionsfrom the outside to the inside and thereby enhance the oxygen partialpressure 400-fold from 0.1 MPa (1 bar) to 40 MPa (400 bar). The partialpressure is readily varied by varying the applied voltage.

Texturing Bulk High Temperature Superconductors

A number of techniques are available for orienting and texturing filmsof high temperature superconductors. However, for bulk superconductors,the only viable technique is that of Murakami (Jap. Jour. Appl. Phys.vol. 28,1189-94, 1989). The method is generally designated asQuench-and-Melt-Growth (QMG) and is disclosed in U.S. Pat. No. 5,278,137(Jan. 11, 1994), issued to Morita et al. There is much activity in thisarea and there are many more recent publications on the subject. The QMGmethod and its many derivatives essentially heat a material to about1400 C., quench it, reheat it to 1100 C. and then slowly cool it. QMG isa crystal growing, rather than a mechanical deformation technique.

OBJECTS AND ADVANTAGES

Accordingly, one object and advantage of my invention is to provide amethod of hot pressing and uniaxially deforming oxide ceramics andsimilar materials, such as oxide high temperature superconductors, whoseequilibrium chemical composition varies with the composition of thegaseous atmosphere with which they are in contact.

Another object of the invention is to be able to do this over a widerange of temperatures and gas pressures.

Still another object is to perform the process in different gaseousatmospheres, whose oxidizing power can vary from chemically reducing tohighly oxidizing, such as pure Argon on one hand and a mixture of 80%argon and 20% oxygen on the other.

A further object is to be able to apply large forces during thedeformation.

Yet a further object is to be able to control these forces so that thedeformation can take place at fast rates--a few seconds, or at muchslower rates tens of minutes or even hours.

Still a further object is to minimize the amount of potentially reactivegases that are present in the equipment at any one time

Still further objects and advantages will become apparent from aconsideration of the ensuing description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 sketches the basic principle of the first part of the invention.It shows a cross-sectional view of a sealed bellows capsule. Massive endplates convert the applied isostatic pressure into a uniaxialcompressive force on a workpiece. All parts have circular symmetry.

FIG. 2 depicts the second and third parts of the invention, to wit: theaddition of a gas supply tube and of gas passages to the interior of thecapsule. This makes it possible to compress the workpiece bydifferential pressure and to implement a controlled atmosphere. Thenon-deforming porous ceramic pusher plates also facilitate gas access tothe workpiece.

FIG. 3 displays the preferred `Top Hat` capsule design with an extendedtop section to prevent overheating of a sensitive workpiece during thesealing of the capsule.

FIG. 4 shows a top view of the capsule of FIG. 3, with the holes thataccept a spanner wrench for mounting the capsule in the bottom of theHIP vessel by its gas supply tube, and with a threaded hole forinserting a lifting bolt.

FIG. 5 presents a plot of temperature (in mV), HIP pressure, capsulepressure, and differential pressure (in kps) against time (in minutes)for a compression cycle.

REFERENCE NUMERALS

10 basic capsule assembly

12 workpiece

14 stemmed capsule assembly

16 `Top Hat` capsule assembly

20 lower anvil

22 lower bellows section

24 lower diffusion barrier

26 modified lower anvil

28 gas passages

30 lower porous ceramic pusher disc

32 modified lower bellows section

40 upper anvil

42 upper bellows section

44 upper diffusion barrier

46 upper porous ceramic pusher disc

48 modified upper bellows section

50 hexagonal lug

52 extended upper anvil

54 extended upper bellows section

56 spanner wrench holes

58 threaded blind hole

60 tubular stem

62 tube bore

90 circumferential bellows weld

91 tube-anvil weld

92 bellows-anvil welds

93 lug weld

95 top hat weld

EMBODIMENTS--DESCRIPTION

My invention combines three components from distinct arts in a novelway. The first (A) is a method of uni-axially compressing a workpiece ina hot isostatic press. The second (B) is to be able to compress at ratesthat do not depend on the capacity of the available compressors. And thethird (C) is the control of the chemical composition of the atmospheresurrounding the workpiece.

FIG. 1 sketches the basic implementation of the first part of myinvention, namely how to use an isostatic press (either hot or cold) toapply uni-axial compressive forces to a workpiece that is essentiallyfree to move in all directions perpendicular to that compressive force.The figure shows a cross-sectional view of a basic capsule 10 containinga pill-shaped workpiece 12 between identical massive metallic anvils--alower anvil 20 and an upper anvil 40. The capsule also has identicalmetallic bellows sections--a lower bellows section 22 and an upperbellows section 42. Identical contamination or diffusion barriers, alower diffusion barrier 24 and an upper diffusion barrier 44, preventadhesion or diffusion between workpiece 12 and lower anvil 20 or upperanvil 40, respectively. The bellows sections 22 and 42 are joined bcircumferential weld 90. The entire assembly has circular symmetry.

As in all of the following capsule designs, the detailed constructionand assembly of capsule 10 is governed by economic factors that aredetermined by the number of identical pieces that will be required. Asanyone skilled in machine shop economics will recognize, there are costtrade-offs between identically performing welded and machinedassemblies. Thus, adjoining lower anvil 20 and lower bellows section 22,as well as adjoining upper anvil 40 and upper bellows section 42, can bemachined out of one solid piece of plate and then dished to angle thebellows section. Or they can be manufactured from stock of differentthickness by machining, stamping, or spinning and then joined bywelding. Or a thin shell could be stamped, formed, or dished in theoutline of the entire lower half of capsule 10, including lower bellowssection 22, while providing a recess to contain lower anvil 20. Thisoption removes the requirement that lower anvil 20 be weldable to lowerbellows section 22 and, indeed, opens up many alternative choices forlower anvil 20. In particular, it allows the substitution of ceramics,or of hard or hardened alloys that cannot or should not be welded.Clearly, due to the symmetry of basic capsule 10, `upper` and `upper`are completely interchangeable. Any optional modifications discussed forthe lower half apply equally well to the upper half of capsule 10.

FIG. 2 adds the second and third parts of my invention, an independentgas connection from an external source to the interior of the capsule.In this stemmed capsule assembly 14, a tubular stem 60, made fromhigh-pressure tubing, is welded at weld 91 into a modified lower anvil26. A tube bore 62 communicates with 28 in modified lower anvil 26. Wehave also a lower anvil 26. We have also added a lower porous ceramicpusher disc 30 in place of lower diffusion barrier 24 (in FIG. 1)between modified lower anvil 26 and workpiece 12. Lower porous ceramicpusher disc 30 assumes the function of lower diffusion barrier 24,provided it is made of, or coated with, a suitable material to avoidcontaminating workpiece 12, as will be discussed later. The lower porousceramic pusher disc 30 also allows gas to reach the surface of workpiece12 from gas passage 28 in modified lower anvil 26. The upper porousceramic pusher disc 46 replaces upper diffusion barrier 44 (in FIG. 1)and performs the identical functions between workpiece 12 and upperanvil 40, as does lower porous ceramic pusher disc 30 between workpiece12 and modified lower anvil 26. In stemmed capsule assembly 14 metallicupper and lower parts are no longer interchangeable, except for themodified bellows sections 32 and 48. Modified lower bellows section 32and modified upper bellows section 48 are attached to their respectiveanvils--modified lower anvil 26 and upper anvil 40--by identical welds92. A hexagonal lug 50 is welded to upper anvil 40 at lug weld 93 topermit mounting an dismounting the capsule inside the HIP vessel bytightening and loosening a conventional sealing gland at the bottom ofits tubular stem.

Mounting a stem-equipped (or `stemmed`) capsule and simultaneouslyconnecting it to a conventional high-pressure gas feed-through,preferably in the bottom plug of the HIP pressure vessel, is mostreadily done with a conventional sealing gland that combines these twofunctions. As anyone versed in the art will recognize, there are manywell-known ways to achieve this functionality. One of the simplest onesis to weld a short cylindrical gland near the bottom of the stem, withjust enough of the stem protruding from the gland to allow the machiningof a fine male thread, typically size 1/4-28 UNF. The threaded portioncan then be screwed into a matching female thread at the exit port fromthe high-pressure gas feed-through. The gland is given an O-ring grooveto accommodate an elastomeric O-ring to seal against a flat, polishedsurface on the bottom plug. However, this is not a unique solution andmany other gland designs and sealing materials, such as soft coppergaskets, can also be used.

The independent gas connection to stemmed capsule assembly 14 providesthe somewhat counterintuitive benefit in that workpiece 12 can becompressed very quickly by simply venting some of the gas from capsule14.

FIG. 3 shows our most preferred `Top Hat` capsule assembly 16. Theextended upper anvil 52 greatly eases the sealing of capsule assembly16. An extended upper bellows section 54, together with the longer andthinner heat conduction path in extended anvil 52, reduce the danger ofoverheating a sensitive workpiece 12 during the final sealing of capsule16 at tip hat weld 95. The internal components of `Top Hat` capsuleassembly 16, as well as modified lower anvil 26, tubular stem 60, andthe use of a sealing gland assembly are unchanged from the lesselaborate stemmed capsule assembly 14. For the Top Hat capsule 16, theextended upper bellows section 54 is preferably manufactured by spinninga thin sheet of material.

FIG. 4 gives a top view of `Top Hat` capsule assembly 16, indicatng thelocation of the blind spanner wrench holes 56 that permit tightening andloosening the sealing gland. A blind threaded hole 58 in the center ofextended upper anvil 52 allows the attachment of a lifting bolt forremoval of capsule 16 from the HIP furnace.

FIG. 5 is a plot of temperature T (in mV from a Pt-PtRh thermocouple andof various pressure (in kpsi) against (in minutes) during a compressioncycle. The pressures are the pressure of the HIP fluid, P1, as measuredby a gauge, G1, and the pressure of the capsule fluid, P2, as measuredby a gauge, G2. The differential pressure DP, defined as P1-P2, iscomputed by the instrumentation to a resolution of better than 0.1% offull scale. The interpretation of the details of FIG. 5 will be given inthe operations section.

Operation

As previously mentioned, my invention provides a method and apparatus touniaxially compress and thereby deform solid materials whose stablechemical composition depends on the composition of their gaseousenvironment. It should be noted that the term `pressure`, unfortunately,appears in two very different contexts. The first is used tocharacterize compression and deformation and is the force applied perunit area. It is a quantity that we can control and manipulate.

The second usage is to describe the pressure of the `atmosphere` thatsurrounds the materials which with we are working, as well as thepartial pressures of various components of that atmosphere. We cancontrol the pressure and chemical composition of this atmosphere.However, the materials themselves establish what is variously calledvapor pressure, decomposition pressure, or thermodynamic equilibriumpressure. That pressure is a specific property of each material. Itvaries (increases) with temperature, but has different values for eachchemical compound or composition. Most of the compositions of interestin this work are oxides and, for most practical purposes, the `vapor` ofinterest is oxygen gas.

If we can make the partial pressure of oxygen, i.e. the total gaspressure multiplied by the percentage of oxygen in the atmosphere, equalto the equilibrium vapor pressure, the material is stable. If thepartial pressure is less than the equilibrium pressure, the materialwill lose oxygen and its composition will change. Similarly, if thepartial pressure is higher than the equilibrium pressure, the materialwill gain oxygen. The rates at which these changes take place, i.e. thekinetics of the reaction, can be quite slow, so that the quantities of`reaction products` can be quite small. Nevertheless, small impuritiescan have significant effects, either detrimental or beneficial, for thekinds of materials of interest in this work.

To recapitulate, this invention manipulates the compressive force on asolid material, while at the same time controlling the composition ofthe gas in the atmosphere surrounding the solid and, thus, the chemicalstability, or thermodynamic state of the solid. The steps to accomplishthese operations for the preferred embodiment of my invention are:

(a) preparing the workpiece,

(b) loading and sealing the capsule,

(c) loading, pressurizing and heating the HIP

(d) compressing the workpiece, and

(e) cooling and unloading the HIP.

(a) Sample Preparation

Sample preparation usually consists of cold pressing a measured quantityof commercially available powder in a cylindrical die to form a shortright cylinder or `pill`. Exposure to atmospheric moisture can bedetrimental to many sensitive oxide materials. It is best, therefore, tostore, weigh, and load the powders under the dry conditions of a glovebox. Similarly, any handling and storage of the pill should be doneunder dry argon gas. Typical sizes for the pills used with our equipmentare a diameter of 10 to 25 mm and a height of 2 to 7 mm. However, thesesizes simply result from the interior dimensions of the availableapparatus and are not an intrinsic limitation imposed by safetyconsiderations. There is no reason why the above dimensions could not bescaled up by a factor of 10. Of course, the equipment would beconsiderably more expensive, but it does exist. Modifications toaccommodate a capsule gas feed-through into a HIP vessel would onlyrepresent a minor cost increase.

(b) Encapsulation

the outer capsule components must be fabricated from a weldable,oxygen-tolerant material. Three nickel alloys, trade-marked andavailable from the International Nickel Company, Inc. are suitable. Wehave used pure nickel (alloy 200), and Inconel 600 for the basic capsuleassembly 10. For the stemmed capsule assemblies 14 and 16, thenickel-copper alloy, Monel 400, is most preferred, particularly sincehigh, pressure tubing in the same ally can be obtained from AutoclaveEngineers Group (Erie Pa.).

In the preferred embodiment workpiece 12 is placed between two porousceramic pusher discs 30 and 46. The material of these discs must becompatible with the workpiece. That is, there should be nocontamination, diffusion, or chemical interaction between the discs andthe workpiece. Ground-to-size porous alumina disc can be obtained fromBolt Technical Ceramics, Inc. in Conroe Tex. To further reduce anyinteraction between the alumina and typical high temperaturesuperconductors, the alumina can be coated wit an yttria (yttrium oxide)spray and then baked. This material is available in aerosol form fromZYP Coatings, Inc. in Oak Ridge, Tenn. Pure yttria ceramic disks areeven more preferred for chemical compatibility and can be obtained fromCustom Specialty Ceramics, Inc. in Arvada, Colo.

the capsules are assembled by welding. The only difference is in thelocation of the final weld closure. For capsules 10 and 14 (FIGS 1 and2), the final assembly is made at circumferential bellows weld 90between the upper and lower bellows sections. This makes it difficult tokeep the workpiece from overheating. For the preferred `Top Hat` capsuleassembly 16 (FIG. 3), the final closure is made at top hat weld 95,which is further away from workpiece 12.

(c) Pressurization-Heating cycle

The Hot Isostatic Press used is a commercial 1980's model `Mini-HIPper`,manufactured by Conaway Pressure Systems, upgraded with anoxygen-tolerant `Hoskins` furnace and with modem pressure andtemperature controllers and modified by adding a high-pressure gasfeed-through in the bottom plug of the pressure vessel.

Installation of the stemmed assemblies 14 and 16 into the HIP isfacilitated by provisions for wrenching. Assembly 14 accepts a socketwrench and assembly 16 a spanner wrench. A fine male machine thread atthe bottom of stem 60 permits proper tightening of the O-ring seal in aconventional sealing gland. The preferred material for the O-ring issilicone rubber. There is no concern about overheating the rubber, sincethe entire sealing gland is in good thermal contact with the massivebottom plug of the HIP.

the primary concern in pressurizing and subsequent heating of theapparatus is to keep the capsule pressure, P2, from exceeding the vesselpressure, P1. Capsules 14 and 16 and the sealing gland assembly weredesigned for P1 always being equal to or larger than P2. If theseconditions should be reversed, the capsule could balloon and itscontents could become rearranged. A pressure in the wrong directioncould also cause the sealing gland assembly to leak and its associatedfine machine thread to fail.

One cylinder (or cylinder bank), S-1, supplies pure argon, and anothercylinder, S-2, supplies the special gas mixtures required by theprocess, such as 20% oxygen in 80% argon. For economic reasons it isdesirable t use only a single compressor for both gas supplies. In thatcase the initial filling of the entire system is done from the processgas supply, S-2. It is important that the initial gas inrush into thesystem be throttled, since the fluid circuits to the HIP vessel and tocapsules 14 or 16 have very different flow impedances. This is done byhaving a throttle valve in parallel with a full flow valve and by beingable to observe the difference between P1 ad P2 with great resolution,using a matched pair of 200 MPa (2000 bar or 30,000 psi) strain gaugepressure transducers, made by Dynisco in Sharon, Mass. The associatedinstrumentation allows observing pressure differences with a resolutionof better than 0.02 Mpa (0.2 bar, 3 psi).

At a predetermined pressure, the two gas systems are sealed off fromeach other and further pressurization of the HIP vessel can be done withgas from the HIP gas supply. This and the subsequent heating cycle therequire a passive or automatic pressure equalizer int he form of anequal-area freely floating (1:1) piston. Its function is to keep thedifferent gas compositions in the HIP and capsule systems from mixing,while maintaining them at the same pressure. It can be obtained fromHigh Pressure Equipment Company in Erie, Pa. Alternatively, and whenworking at shorter time scales, mixing between gases of differentcomposition can be slowed down by replacing the equalizer with a longlength of 2.11 mm I.D. high pressure tubing. In such an arrangement,mixing will be limited by the slowness of gas diffusion in a long, smalldiameter tube. However, this arrangement requires a detailed analysis ofthe relative magnitudes of the heated and `dead` volumes of the capsuleand vessel fluid circuits, in order to assure that any flow is out ofthe capsule so that the composition of the capsule fluid remainsunchanged.

Our standard operating procedure is to initially use a throttling valveto fill the entire system to bottle pressure with process gas, S-2.Next, we open a parallel full flow valve and use a Haskel™ air-driven,high-pressure gas compressor to raise the pressure to 40 or 60 MPa (400to 600 bar or 6000 to 9000 psi). It is possible to switch from theprocess supply, S-2, to the inert argon supply, S-1, before reaching 60MPa. This depends on the volume of the equalizer and on the desired gascompositions. Finally, depending on the desired ultimate pressure andtemperature, the compressor is stopped, all supply valves are closed,the HIP furnace is turned on and ramped up under microprocessor control.This, of course, raises both pressure and temperature at the same time.

Once the desired pressure and temperature conditions are reached, thepressure equalizer is disabled by closing a valve in series with it adthe differential pressure is observed to make sure that there are noleaking valves.

(d) Compression

Compression of the contents of the capsule is accomplished in anextremely simple and uncomplicated manner by opening a venting valve onthe capsule lines and releasing some of the gas from the capsule. Therate of compression and consequent rate of deformation of the contentsof the capsule are simply controlled by the degree of opening of thatvalve. The whole process can be done in less than a minute, in about 8minutes, as shown in FIG. 5, or over much longer time intervals. Thereis no punch and die that might bind (as mentioned in the literature).The drop in the value of the pressure difference apparent between 238and 246 minutes (FIG. 5) is probably caused by plastic flow in thecapsule, leading to a reduction in volume and consequent increase incapsule pressure.

(e) Cool-Down

After the desired holding interval, the furnace can either be `scrammed`for rapid cool-down, in effect quenching the workpiece (as seen by theblip at 247 minutes), or it can be programmed to cool at any desiredslower rate.

Conclusions, Ramifications, and Scope

Accordingly, it can be seen that my invention combines some of the moreuseful aspects of conventional hot pressing and of hot isostaticpressing. It provides texturing and densification, both of which tend toimprove materials properties and performance. It offers a new way of hotworking, or forging for materials that are sensitive to the chemicalcomposition of their gaseous environment by fully controlling thatenvironment. At the same time, my invention can carry out thedeformation over a wide range of time scales, without requiringexpensive high-speed, high-volume compressors. The ability to vary therates is a significant advantage, because the plastic deformation andflow exhibited by many materials in response to applied forces is oftenquite rate dependent. It is, therefore, difficult to a priori predictoptimum deformation schedules. It is obviously useful to have anapparatus that allows sufficient variation in the rate parameters todetermine the optimum process.

My invention also has the advantage that, if corrosive or hazardousgases are needed to maintain chemical equilibrium, their amounts can beminimized to just the volume of the capsule. At the same time the amountof the, sometimes expensive, alloys that can handle the corrosivematerials is limited to the capsule and its connecting tubing.

An incidental advantage of my procedure is that the presence of the gasrequired for chemical stability does not prevent full compaction of theworkpiece, as it would with a canned HIP procedure.

My solution to providing uniaxial compression with the anvils and pusherdiscs inside a capsule does not suffer some of the drawbacks ofconventional punch and die arrangements. In particular, we do not haveto be concerned with binding and jamming of the punch.

Although the above description contains many specificities, these shouldnot be construed as limiting the scope of the invention, but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Various other embodiments and ramifications arepossible within it's scope. For example, the ceramic pusher discs do nothave to have plane pusher surfaces. They could be convex or concave todeform the workpiece into a more complex shape than simple disc.

Nor need the material of the workpiece be limited to just oxides.Indeed, almost any inorganic compound or composition, as well as someorganic ones, could be candidates for hot, cold, or warm forging by mydifferential pressure procedure. Finally the use of my apparatus neednot be limited to only providing thermodynamically stable products.There is much interesting work that can be done with metastable,quenched or partially transformed phases. The apparatus is, of course,equally able to function in such a regime of instability.

Thus the scope of the invention should be determined by the appendedclaims and their legal equivalents, rather than by the examples given.

I claim:
 1. An apparatus for forging, or mechanically deforming a solidbody, comprising:(a) a sealed capsule with flexible segments, containingsaid solid body, (b) a tubular stem, one end of said tubular stem beingconnected to said sealed capsule, said tubular stem providing a fluidpassage to the interior of said capsule, and said tubular stem alsosupporting said sealed capsule in the controlled temperature region ofthe pressure vessel of a conventional Hot Isostatic Press, or HIP, (c) ameans of connecting and attaching the other end of said tubular stem tothe outlet of a conventional high-pressure fluid feed-through into saidpressure vessel, (d) a fluid connection from the exterior of saidpressure vessel through said feed-through to the interior of saidtubular stem and from the interior of said tubular stem to the interiorof said sealed capsule, (e) interconnected fluid supply vessels, valves,piping, compressors and fluid handling equipment to provide separate andindependent supplies of fluids at elevated pressures: a supply ofcapsule fluid to the interior of said capsule, and a supply of pressurevessel fluid to the interior of said pressure vessel, whereby thepressure and composition of said fluids are manipulated independently ofeach other, (f) fluid passages within said capsule and within thecontents of said capsule, whereby said capsule fluid communicatesthroughout said capsule and contacts substantially the entire surface ofsaid solid body, (g) combined means of equalizing the pressure of saidfluids and of preventing mixing of said pressure vessel fluid with saidcapsule fluid, (h) at least one pair of rigid, plate-like shaped anvils,said anvils forming an integral part of said capsule, said anvils beinglocated pairwise on opposite sides of said solid body, one side of saidanvils being in contact with said solid body and with said capsulefluid, and the other side of said anvils being in contact with saidpressure vessel fluid, and said anvils thereby being capable oftranslating any pressure difference between said pressure vessel fluidand said capsule fluid into substantially uni-axial compressive forceson said solid body, and (i) a means of releasing some of said capsulefluid in a predetermined manner, thereby reducing the fluid pressureinside said capsule and generating a predetermined pressure differencebetween the exterior and the interior of said capsule,whereby a force isapplied to said solid body by said anvils, thereby compressing anddeforming said solid body at predetermined conditions of strain rate, oftemperature, and of the pressure, chemical composition and chemicalactivity of its fluid environment, so that the thermodynamic state ofsaid solid body within said surrounding capsule fluid remains definedduring the deformation process.
 2. The apparatus set forth in claim 1(part g), wherein said pressure equalization means without mixing isimplemented by an equal-area floating piston.
 3. The apparatus set forthin claim 1 (part g), wherein said pressure equalization means withoutmixing is implemented by a length of small diameter high pressure tubingof at least 1 m in length.
 4. The apparatus set forth in claim 1 (parth), wherein the design of said anvils is implemented in cylindricalgeometry.
 5. The apparatus set forth in claim 1 (part h), wherein thesurfaces of said anvils in contact with said solid body are covered witha refractory coating, said coating serving as a diffusion barrier andthereby preventing adhesion to and chemical changes in said solid body.6. The apparatus set forth in claim 5, wherein said refractory coatingconsists of yttria.
 7. The apparatus set forth in claim 1 (part h),wherein each set of said pairwise anvils consists of both metallic andrefractory non-metallic components, selected, in combination, to providerigidity and a barrier to diffusion across the interface between saidanvils and said solid body.
 8. The apparatus set forth in claim 7,wherein said components contain interconnected channels or pores therebyfacilitating the free entry, exit and circulation of said capsule fluidto the surfaces of said solid body.
 9. The apparatus set forth in claim7, wherein the material for said non-metallic components is selectedfrom a group of refractory oxides, the group consisting of magnesia,alumina, titania, yttria, zirconia, and ceria.
 10. The apparatus setforth in claim 1 (part i), wherein a precision metering valve isinstalled to partially release said fluid in said capsule at apredetermined rate, thereby controlling the rate of application of saidcompressive and deforming force and thereby deforming said solid bodyover time scales ranging from seconds to hours.
 11. The apparatus setforth in claim 1 (part h), wherein a formed shell capsule supports andlocates a pair of anvils on opposite sides of said solid body.